“The cable companies have said it’s their worst nightmare to be classified as a telecommunications service.”

ISPs such as America Online and Earthlink have been hungering after high-speed cable hookups, which are must faster than dial-up Internet service.

They have been arguing that AT&T must open its high-speed connections just as local phone companies must open their networks to competitors.

But AT&T has spent $120 billion buying cable companies to create its own network for local telephone and high-speed Internet service.

It currently has an exclusive deal with the Internet service provider Excite@Home, which it partially owns, to deliver high-speed service over those lines.

AT&T general counsel Jim Cicconi pronounced himself “pleased” with the decision, “because it clarifies decisively the limits of local authority when it comes to the provision of high-speed Internet access over cable.

Investors also appeared to be pleased, bidding up AT&T stock $1 to $36.

AT&T has been looking for good news to trumpet for some time.

The company has had to lower earnings projections; its stock has been sluggish, and its costly bet on remaking itself from a stodgy old phone company into a cable giant ready to do battle in the digital age has yet to pay off.